The present invention relates to megasonic cleaning systems and more particularly to a method and apparatus for determining the optimum frequency at which to drive the megasonic transducer.
It is well-known that sound waves in the frequency range of 0.4 to 2.0 megahertz (MHZ) can be transmitted into liquids and used to clean particulate matter from damage sensitive substrates. Since this frequency range is predominantly near the megahertz range, the cleaning process is commonly referred to as megasonic cleaning. Among the items that can be cleaned with this process are semiconductor wafers in various stages of the semiconductor device manufacturing process, disk drive media, flat panel displays and other sensitive substrates.
Megasonic acoustic energy is generally created by exciting a crystal with radio frequency AC voltage. The acoustical energy generated by the crystal is passed through an energy transmitting member and into the cleaning fluid. Frequently, the energy transmitting member is a wall of the vessel that holds the cleaning fluid. The crystal and its related components are referred to as a megasonic transducer. For example, U.S. Pat. No. 5,355,048, discloses a megasonic transducer comprised of a piezoelectric crystal attached to a quartz window by several attachment layers. The megasonic transducer operates at approximately 850 KHz. Similarly, U.S. Pat. No. 4,804,007 discloses a megasonic transducer in which energy transmitting members comprised of quartz, sapphire, boron nitride, stainless steel or tantalum are glued to a piezoelectric crystal using epoxy.
It is also known that piezoelectric crystals can be bonded to certain materials using indium. For example, U.S. Pat. No. 3,590,467 discloses a method for bonding a piezoelectric crystal to a delay medium using indium where the delay medium comprises materials such as glasses, fused silica and glass ceramic.
In ultrasonic and megasonic cleaning systems, the crystal used in the transducer must be driven at a frequency that excites the natural anti-resonant frequency of the crystal in the chosen mode of operation, and which is compatible with the other components used in the transducer and the overall cleaning system. Furthermore, when the cleaning system is in operation, the driving or excitation frequency may need to be adjusted slightly because of temperature changes or other variations in the cleaning system. Many different techniques exist for tuning a transducer (i.e. for selecting and/or maintaining the excitation frequency). For example, prior art circuits that use a phase locked loop to make adjustments to the excitation frequency are known. However, such circuits are relatively complicated and include circuitry that must be added to the transducer system for the sole purpose of tuning the transducer. Most of these prior art systems also include hardware, such as a directional coupler and an analog to digital converter/sample hold circuit, for measuring the reflected and forward power. However, in the prior art these hardware components are not used for taking measurements that are utilized in a numerical method for tuning the transducer.
Briefly, the present invention is a method and apparatus for selecting the optimum frequency at which to drive the megasonic transducer in a megasonic cleaning system which does not require the use of a phase locked loop circuit. The method of the present invention uses a numerical method to tune the megasonic transducer. Furthermore, the raw data for the numerical method is generated by circuit components that are used in the cleaning system for purposes other than tuning the transducer. As used herein, the phrase xe2x80x9ctuning the transducerxe2x80x9d refers to the process of selecting the optimum excitation frequency at which to drive the megasonic transducer.
In the method of the present invention, a plurality of frequency values that span a frequency range containing an optimum frequency for driving a piezoelectric crystal are generated by a microprocessor. The reflection coefficient xe2x80x9cxcfx81xe2x80x9d at each of these frequency values is determined, where xe2x80x9cxcfx81xe2x80x9d is the reflected power divided by the forward power. This data is then fitted to a function using regression techniques to obtain the coefficients of the function. Using a third degree polynomial for the function works well in the technique.
The first derivative of the function is then calculated by the microprocessor and the roots of the first derivative equation are determined. The optimum frequency is selected from the set of roots, generally as the real root that is a minima within the examined frequency range. Variations of this method include using other functions in place of the third degree polynomial, and/or replacing the reflection coefficient with just the reflected power value.
The piezoelectric crystal used in the megasonic cleaning system is capable of generating acoustic energy in the frequency range of 10.0 KHz to 10.0 MHz when power is applied to the crystal. In a preferred embodiment, the attachment layer is comprised of indium and is positioned between the resonator and the piezoelectric crystal so as to attach the piezoelectric crystal to the energy transmitting member. A first adhesion layer comprised of chromium, copper and nickel is positioned in contact with a surface of the piezoelectric crystal. A first wetting layer comprised of silver is positioned between the first adhesion layer and the bonding layer for helping the bonding layer bond to the first adhesion layer. A second adhesion layer comprised of chromium, copper and nickel is positioned in contact with a surface of the resonator. A second wetting layer comprised of silver is positioned between the second adhesion layer and the bonding layer for helping the bonding layer bond to the second adhesion layer. Of course the method and apparatus for selecting the optimum frequency at which to drive the megasonic transducer can be used with other types of transducers, including transducers that do not have an indium layer.